The human rights organization cited "multiple interviews with credible sources" in asserting that Nigerian security forces had four hours of notice about the April 15 attack by Boko Haram in northeastern Nigeria.

"The fact that Nigerian security forces knew about Boko Haram’s impending raid, but failed to take the immediate action needed to stop it, will only amplify the national and international outcry at this horrific crime,” said Amnesty International spokesman Netsanet Belay.

Women holding signs take part in a protest demanding the release of abducted secondary school girls from the remote village of Chibok, in Lagos. The Islamist militant group Boko Haram has claimed responsibility for the abduction of more than 200 schoolgirls during a raid last month. (Akintunde Akinley/Reuters)

"It amounts to a gross dereliction of Nigeria’s duty to protect civilians, who remain sitting ducks for such attacks. The Nigerian leadership must now use all lawful means at their disposal to secure the girls’ safe release and ensure nothing like this can happen again."

More than 300 girls were initially abducted, with 53 of them escaping later. At least 276 are still missing.

The Nigerian military's failure to find the girls has drawn international attention to an escalating Islamic extremist insurrection that has killed more than 1,500 people so far this year.

The mass kidnapping has focused the world's attention on Boko Haram, and on the many civilian victims of the extremists.

President Goodluck Jonathan said at an economic forum on Thursday: "I believe that the kidnap of these girls will be the beginning of the end of terror in Nigeria."

But his government stands accused of being slow to mount operations to rescue the girls.

Amnesty says local civilian patrols were the first to raise the alarm about the impending attack, alerting officials including the local state governor and senior military commanders.

Two seniors officers in Nigeria's armed forced confirmed the military was aware of the planned attack prior to the calls received from local officials, according to Amnesty. One officer, whose name was not given, was quoted by Amnesty as saying that soldiers are afraid to go to the battlefronts.

The group says an inability to muster troops and fear of engaging with better equipped forces prevented forces from being deployed. Amnesty International’s requests for a reaction from the military headquarters in Abuja have gone unanswered.

Boko Haram, which wants to impose Islamic law on Nigeria, has threatened to sell the girls. The group's name means "Western education is sinful."

Chibok residents staged a street protest Friday to press Borno's government to do more to find the missing girls.

British, U.S. experts join search

Meanwhile, British security experts on Friday joined the Nigerian and American search as an international effort began taking hold.

The British team, which arrived in Lagos Friday, is expected to work closely with U.S. officials and agents in the search for the missing girls, the British government said, as Boko Haram militants continued to stage attacks in northeastern Nigeria.

China and France have also promised help, and the deputy prime minister of Spain told reporters in Madrid on Friday that her government had decided to make available a specialist police team to assist, if Nigeria approves.

Britain said its aim was not only to help with the current crisis but to defeat Boko Haram.

"The team will be considering not just the recent incidents but also longer-term counter-terrorism solutions to prevent such attacks in the future and defeat Boko Haram," the Foreign & Commonwealth Office said in a statement Friday.

A local government official confirmed on Friday the Islamic extremists bombed a bridge linking the town of Gamboru to the Borno state capital — the headquarters of the Nigerian military offensive — reportedly as they retreated following an earlier attack on Gamboru's main market, where at least 50 bodies have since been discovered from the debris of burned shops.

But communications with the remote town are difficult and it was not immediately possible to reconcile conflicting accounts of when the bridge was bombed. One account said Monday while another said Thursday.

Local traders in Gamboru said Friday that their businesses were suffering, with trailers and heavy trucks now stranded on either side of the damaged bridge.

The bombing of the bridge also prevents army convoys reaching Gamboru while leaving the way open for the insurgents to escape across a strategic bridge into neighbouring Cameroon — a bridge leading into mountains where the militants are known to have hideouts in caves.

Boko Haram has staged many attacks in northeastern Nigeria over the years, a campaign of bombings and massacres that has intensified in recent times despite a strong military presence there. Since May 2013 there has been a state of emergency in three northeaster Nigerian states wracked by Boko Haram violence.

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